1. Technical Field
This invention relates to signal processing systems. In particular, this invention relates to compensating non-uniformity among microphones in a multiple microphone system.
2. Related Art
Microphones used in signal processing systems often have non-uniform characteristics. For example, the microphones in a hands-free voice command or communication system in an automobile may detect the same speech signal, but nonetheless produce very different microphone output signals. Non-uniform microphone characteristics may result from variations in the microphone fabrication process, from changes arising in the microphones from age, use, temperature, humidity, altitude, or from other factors. Non-uniform microphone characteristics may result in non-uniform frequency response between microphones, reduced signal strength and sampling accuracy, inconsistent sampling of sound signals, and generally reduced system performance.
One past attempt to compensate for microphone non-uniformities relied on pre-configuring digital filters with invariant initial settings to process the microphone signals. The initial settings depended upon the frequency response of the respective microphone and an extensive preliminary measurement and analysis phase. In the analysis, an optimally placed speaker output an audio signal with known characteristics. The microphone signals capturing the audio signal were then analyzed to determine optimum filter settings for each digital filter. The communication system used the same filter settings during its operational lifetime.
The filter settings were also determined based on the estimated or predicted conditions in which the communication system would operate. Thus, the initial measurements and analysis were extensive, but needed to accurately model the conditions in which the communications system would operate. Regardless, age, use, temperature, humidity, altitude, or other factors temporarily or permanently altered microphone characteristics, including frequency response, after the initial determination of the filter settings. Accordingly, the performance of the communication system degraded over time.
Therefore, a need exists for an improved system for compensating for microphone non-uniformity.